An optical disk utilizing a thin rare-earth transition metal alloy film as a recording medium is in a practical application stage as a digital memory. When information recorded in the optical disk is to be reproduced, usually a linearly polarized light generated by a semiconductor laser is irradiated on a recording medium. After the linearly polarized light is reflected off the recording medium, an amount of rotation of a polarization plane thereof is detected by an analyzer and converted into light intensity.
The principles of a reproducing method as described above, wherein the so-called Kerr effect is used, are described hereinbelow.
When the linearly polarized light generated by the semiconductor laser is reflected off the recording medium, the polarization plane thereof is rotated to the right or the left corresponding to a differing perpendicular magnetization direction of the recording medium. A reflected light R.sub.11 (shown in FIG. 8) that is reflected off the recording medium includes binary information recorded in the recording medium by transposing the binary information into a rotational direction of the polarization plane. The reflected light R.sub.11 is directed to an analyzer 31 and is split into a detected light D.sub.11 and a detected light D.sub.12 in the analyzer 31 according to a difference in the rotational direction of the polarization plane of the light R.sub.11. A photodetector 32 and a photodetector 33 convert the detected light D.sub.11 and the detected light D.sub.12 respectively into electric signals, thereby generating a reproduced signal S.sub.11 and a reproduced signal S.sub.12 respectively.
Suppose that a specified perpendicular magnetization direction of the recording medium is designated by (+) and an opposite perpendicular magnetization direction thereof is designated by (-). Also, suppose that .gamma. is an incident light vector, .alpha. is a reflected light vector reflected off a recorded bit magnetized in the (+) direction, and .beta. is a reflected light vector reflected off a recorded bit magnetized in the (-) direction. As shown in FIG. 9, if the polarization plane of the reflected light vector .alpha. rotates, for example, by Kerr rotative angle +.theta..sub.k with respect to the incident light vector .gamma., the polarization plane of the reflected light vector .beta. will on the contrary rotate by Kerr rotative angle -.theta..sub.k with respect to the incident light vector .gamma..
Since a polarization direction X and a polarization direction Y of the analyzer 31 are mutually orthogonal, intensities of the reflected light vector .alpha. and the reflected light vector .beta. are respectively split into X components and Y components thereof and are then detected. The photodetector 32 generates the reproduced signal S.sub.11. Here, a high level of the reproduced signal S.sub.11 corresponds to an X component .beta..sub.x of the reflected light vector .beta. and a low level of the reproduced signal S.sub.11 corresponds to an X component .alpha..sub.x of the reflected light vector .alpha.. That is, the high level of the reproduced signal S.sub.11 corresponds to the recorded bit magnetized in the (-) direction.
On the other hand, when the reproduced signal S.sub.11 is at the low level corresponding to the X component .alpha..sub.x of the reflected light vector .alpha., the photodetector 33 generates the reproduced signal S.sub.12 that is at a high level corresponding to a Y component .alpha..sub.y of the reflected light vector .alpha.. Further, when the reproduced signal S.sub.11 is at the high level corresponding to the X component .beta..sub.x of the reflected light vector .beta., the photodetector 33 generates the reproduced signal S.sub.12 that is at a low level corresponding to a Y component .beta..sub.y of the reflected light vector .beta.. The high level of the reproduced signal S.sub.12 corresponds to the recorded bit magnetized in the (+) direction.
As described above, the high level of the reproduced signal S.sub.11 generated by the photodetector 32 corresponds to the recorded bit magnetized in the (-) direction and the high level of the reproduced signal S.sub.12 generated by the photodetector 33 corresponds to the recorded bit magnetized in the (+) direction. The reproduced signal S.sub.11 and the reproduced signal S.sub.12 have a phase difference of half a cycle, and vary inversely with respect to each other. Furthermore, the reproduced signal S.sub.11 and the reproduced signal S.sub.12 achieved in this way largely exclude disk noise since they are not affected by dust particles or the like attached to the disk surface. This is because the reproduced signal S.sub.11 and the reproduced signal S.sub.12 are based on the rotation of the polarization plane of each reflected light. Moreover, the SN ratio may be further improved by supplying the reproduced signal S.sub.11 and the reproduced signal S.sub.12 to a differential amplifier and performing information reproduction based on an output signal of the differential amplifier.
However, the reproduction method based on the Kerr effect described above and normally used for performing magneto-optical recordings has a problem in that a high degree of accuracy is necessary for setting the analyzer 31. The method has a further disadvantage of causing a rise in cost of the reproducing device.
A method of reducing the cost of the reproducing device by not using an analyzer and thereby simplifying the reproducing optical device has already been developed theoretically. This method uses the so-called circular dichroism effect. The circular dichroism effect is a phenomenon whereby the intensity and the phase of a circularly polarized light irradiated on a recording medium and reflected therefrom exhibit anisotropy corresponding to the differing perpendicular magnetization directions of the recording medium.
As shown in FIG. 10, suppose that n.sub.0 is the refractive index of a medium on an irradiated face of a recording medium 34, n.sub.+ is the refractive index of a recorded bit 34a wherein a magnetization direction is directed upwards with respect to the recording medium 34, r.sub.+ is the complex reflectance of the recorded bit 34a, n.sub.- is the refractive index of a recorded bit 34b wherein a magnetization direction is directed downwards with respect to the recording medium 34 and r.sub.- is the complex reflectance of the recorded bit 34b. For example, suppose that there is a right circularly polarized light L.sub.11 irradiated on the recording medium 34 and that an electric field vector of the right circularly polarized light L.sub.11 rotates clockwise facing a light source. A reflected light from the recorded bit 34a becomes a left circularly polarized light L.sub.12 and a reflected light from the recorded bit 34b becomes a left circularly polarized light L.sub.13 having a weaker intensity than the intensity of the left circularly polarized light L.sub.12. A difference in reflected light intensity between the recorded bit 34a and the recorded bit 34b may be expressed by the following formula (1). EQU (r.sub.+).sup.2 -(r.sub.-).sup.2 ( 1)
(If a left circularly polarized light is irradiated on the recording medium 34, the reflected light becomes a right circularly polarized light and the above relationship between the reflected light intensities corresponding to the recorded bit 34a and the recorded bit 34b is reversed). Further, r.sub.+ and r.sub.- may be expressed in terms of n.sub.0, n.sub.+ and n.sub.- as follows. EQU r.sub.+ =(n.sub.0 -n.sub.+)/(n.sub.o +n.sub.+) (2) EQU r.sub.- =(n.sub.0 -n.sub.-)/(n.sub.o +n.sub.-) (3)
However, in a reproduction method using the circular dichroism effect, noise gets included in the reproduced signal since an attempt is made to detect the magnetization direction of the recording medium by transposing it into a difference in reflected light intensity. This is because foreign matter such as dust particles that adhere to the surface of the optical disk affect the reflected light intensity. As a result, there is a greater tendency for signal quality to deteriorate when a reproduction method using the circular dichroism effect is employed than when a conventional reproduction method using the Kerr effect is employed.